Generally, in zoom lenses used as projection lenses in projection display devices, an aperture diameter corresponding to a liquid crystal projector for providing a bright projected image is desired. Compactness of the zoom lens in order to make the projection-type display device small is also desired. In addition, these zoom lenses need to define optical systems that are telecentric, or nearly telecentric, on the reducing side where the display element, such as a liquid crystal display element, is positioned, and that have appropriately large back focal lengths so that there is sufficient room for a color synthesizing optical system (which would operate as a color separating optical system in the reverse direction) for combining information from different display elements.
In addition, in recent years, with these zoom lenses for use in projection display devices, in accordance with the desire to project large images, for example, large video images, at close range to a large screen, wide-angle projection has been desired. Also, during the past several years, there has been strong demand for making projection display devices compact, and in order to do this, the zoom lens itself must be made compact, as well as shortening the overall length of the zoom lens in order to assure projection of a bright image.
These types of zoom lenses are known, as described, for example, in Japanese Laid-Open Patent Applications 2001-91829, 2004-20765, 2000-137165, 2001-100100, and 2001-324677, and 2002-72094. These applications describe five-group zoom lenses. In order to achieve continuous variations in focal length and simultaneously compensate for movement of the image plane that would otherwise occur, three lens groups undergo coordinated movements along the optical axis. In the above mentioned Japanese Laid-Open Patent Applications 2001-91829 and 2004-20765, the lens groups, from the enlarging side, have negative, positive, positive, positive, and positive refractive powers. In the above mentioned Japanese Laid-Open Patent Applications 2000-137165, 2001-100100, 2001-324677, and 2002-72094, the lens groups, from the enlarging side, have negative, positive, positive, negative, and positive refractive powers.
The zoom lenses described in Japanese Laid-Open Patent Application 2001-91829 have an f-number of 2.0 at maximum aperture diameter at the wide-angle end, which is relatively dark, and overall lengths that fail to satisfy the demand of recent years for shortening the lens system. Also, it is desirable that the projection angle, that is, the field angle on the enlarging side, be larger.
The zoom lenses of Japanese Laid-Open Patent Application 2004-20765 do provide for a brighter image with an f-number of 1.73 at the wide-angle end. However, the overall length of the lens system fails to sufficiently satisfy the requirements of recent years of shortening the lens system.
With regard to zoom lenses of Japanese Laid-Open Patent Application 2000-137165, when an f-number of 2.0 is provided at the wide-angle end, the images are relatively dark. In addition, when an f-number of 1.7 at the wide-angle end is provided, the requirements of recent years of shortening the lens system are not adequately satisfied. Furthermore, it is desirable that the picture angle, that is, the field angle on the enlarging side, be larger than the fifty degrees taught in Japanese Laid-Open Patent Application 2000-137165.
With regard to zoom lenses of Japanese Laid-Open Patent Application 2001-100100, when an f-number of 2.0 at the wide-angle end is provided, the images are relatively dark. In addition, when an f-number of 1.84 is provided, the requirements of recent years of shortening the lens system are not adequately satisfied.
With regard to zoom lenses of Japanese Laid-Open Patent Application 2001-324677, the f-number at the wide-angle end is 1.5. However, the requirements of recent years of shortening the lens system are not adequately satisfied. Furthermore, it is desirable that the picture angle, that is, the field angle on the enlarging side, be larger than the forty-eight degrees taught in Japanese Laid-Open Patent Application 2001-324677.
Japanese Laid-Open Patent Application 2002-72094 achieves superior compactness in terms of the overall length of the zoom lenses described, but includes the use of an aspheric surface. Additionally, the zoom lenses are relatively dark with an f-number at maximum aperture at the wide-angle end of 1.73, and it is desirable that a wider picture angle, that is, field angle on the enlarging side be provided.
As can be understood from these conventional examples, it is extremely difficult to shorten the overall length of such zoom lenses while simultaneously achieving a bright image and a large angle of projection of a projected image, which is defined by the field angle, or picture angle, on the enlarging side of the zoom lens.